Printable colour images comprise pixels that indicate an amount of colour. Often these colour pixels have three components, indicating e.g. an amount of red, green and blue. The colour components do not necessarily correspond with the colorants in a printer. If the number of colorants in the printer is the same as the number of colour components, there is a one-to-one relation between the colour components and the colorants in the printer. Often a printer has more colorants to obtain a larger gamut, which is the full set of printable colours. Most common are printers employing cyan (C), magenta (M), yellow (Y) and black (K) ink. Ink here includes liquid ink, wax, and dry ink or toner. In the case of more colorants there are multiple ways to apply the colorants to create a similar colour on a print position, which indicates a position on a receiving medium in the printer where an amount of colorant may be applied. A well known example is the use of gray component replacement (GCR) and under colour removal (UCR), in which parts of cyan (C), magenta (M) and yellow (Y) may or may not or may partly be substituted by black colorant (K). This substitution results in a reduction of ink usage, but the visual quality of a printed document may also be affected. An extensive use of black ink by ink saving methods not only puts pastels and skin tones in risk of developing a grainy appearance, but also increases the visibility of printing artifacts and decreases the vividness of colors. Depending on the contents of a document, or the objects in a document, print quality may be exchanged for ink saving and vice versa. The same applies for other ink saving methods, such as draft mode mode technology in which e.g. only alternate dots of an image are printed.
In U.S. Pat. No. 7,224,487 entitled “Ink reduction error diffusion” a waste colour remover (WCR) is disclosed which analyses the colour data, after a halftoning step, such as error diffusion or dithering, to determine redundant colour data. This module eliminates non-black dots, if multiple dots of colour are intended for the same print position as a black dot. The non-black dots can be eliminated without compromising the output image integrity. Some further particular cases are considered in which removing colorant is not beneficial. Nevertheless, the performance of the method depends strongly on the choice of ink combinations contained in a subset used during the quantization, or halftoning step.
In general, ink saving methods may be applied during the colour management stage of a printing workflow. Depending on the colours involved more or less ink saving may be applied. In U.S. Pat. No. 6,233,061 entitled “Ink reduction device and method for a color inkjet printer” an output colour profile is based on the colour distance between a reduced ink composition of a colour and its full ink composition. This provides an efficient method to reduce the excess colour ink placed on a substrate without impairing the visually distinguishable colour range of a printer.
However, besides colour matching, more print quality aspects are involved when ink saving is applied. These aspects comprise graininess and sharpness of the image, which are related to the placement of dots in the image. A different optimum may be found for different type of images, or type of objects in an image, such as text and pictorial images. A problem exists in compensating various aspects of print quality when a predefined level of ink usage for a type of object is set. An object of the present invention is to maintain visual print quality when controlling the percentage of ink usage.